This invention relates to processes for converting methanol and olefins to high octane liquid fuel. In particular, this invention relates to an integrated system for the production of methyl tertiary alkyl ethers in conjunction with the conversion of oxygenates and olefins to gasoline and distillate fuels.
In recent years, a major technical challenge presented to the petroleum refining industry has been the requirement to establish alternate processes for manufacturing high octane gasoline in view of the regulated requirement to eliminate lead additives as octane enhancers as well as the development of more efficient, higher compression ratio gasoline engines requiring higher octane fuel. To meet these requirements the industry has developed non-lead octane boosters and has reformulated high octane gasoline to incorporate an increased fraction of aromatics. While these and other approaches will fully meet the technical requirements of regulations requiring elimination of gasoline lead additives and allow the industry to meet the burgeoning market demand for high octane gasoline, the economic impact on the cost of gasoline is significant. Accordingly, workers in the field have intensified their effort to discover new processes to manufacture the gasoline products required by the market place. One important focus of that research is new processes to produce high octane gasolines blended with lower aliphatic alkyl ethers as octane boosters and supplementary fuels. C.sub.5 -C.sub.7 methyl alkyl ethers, especially methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) have been found particularly useful for enhancing gasoline octane. Therefore, improvements to the processes related to the production of these ethers are matters of high importance and substantial challenge to research workers in the petroleum refining arts.
It is known that isobutylene may be reacted with methanol over an acidic catalyst to provide methyl tertiary butyl ether (MTBE) and isoamylenes may be reacted with methanol over an acidic catalyst to produce tertiary-amyl methyl ether (TAME). In these etherification processes a problem of major importance is the separation of methanol from the etherification reaction product due to the proclivity of methanol to form a very dilute azeotropic mixture with hydrocarbons and the strong solubility of methanol in both water and hydrocarbons. Due largely to these factors, the cost associated with methanol separation and recycling in the etherification reaction represents approximately 30% of the cost of the total etherification process.
In U.S. Pat. No. 4,684,757 to Avidan et al. the well-known ability of zeolite type catalyst to convert methanol to olefins is utilized by directing unreacted methanol from an etherification reaction to a zeolite catalyzed conversion reaction for conversion to olefin, thereby obviating the need to separate and recycle methanol in the etherification reaction. However, the process of Avidan et al. converts oxygenate feedstock. The process incorporates an alkylation step in one embodiment to produce alkylate as well as C.sub.5 + gasoline and C.sub.5 + ethers.
The process for the conversion of methanol to olefins utilized in the Avidan et al. patent is but one in a series of analogous processes based upon the catalytic capabilities of zeolites. Depending on various conditions of space velocity, temperature and pressure methanol, and lower oxygenates in general, can be converted in the presence of zeolite type catalyst to olefins which may then oligomerize to provide gasoline or distillate or be converted further to produce aromatics.
In another application of zeolite catalysis, at low pressure and high temperature light olefins can be interconverted or redistributed to produce higher olefins rich in isoalkenes.
The feasibility and adaptability of the basic chemistry of zeolite oxygenates conversion to produce useful conversion processes has been the subject of much inventive research activity. Recent developments in zeolite catalyst and hydrocarbon conversion processes have created interest in using olefinic feedstocks for producing C.sub.5 + gasoline, diesel fuel, etc. In addition to the basic work derived from ZSM-5 type zeolite catalyst, a number of discoveries have contributed to the development of a new industrial process, known as Mobil Olefins to Gasoline/Distillate ("MOGD"). This process has significance as a safe, environmentally acceptable technique for utilizing feedstocks that contain lower olefins, especially C.sub.2 -C.sub.5 alkenes. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al. have also contributed improved processing techniques to the MOGD system as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The conversion of olefins to gasoline using a fluidized catalyst bed is the subject of U.S. Pat. application Ser. No. 6,407 to Owen, et al. The above identified disclosures are incorporated herein by reference.
The MOGD process may produce low octane gasoline. This disadvantage requires further downstream processing of the product so produced in order to provide a gasoline product with useful road octane value. Improvement of the process to provide an instant higher octane value gasoline product has been a much sought after objective in that field of art.
Recognizing the limiting problems of the etherification processes to produce MTBE and TAME and the potential that resides in the general area of the chemistry of oxygenate and olefin conversion with zeolites to resolve those problems, several objectives of the instant invention have been established.
First, it is an object of the present invention to provide an integrated process for the production of liquid fuel mixtures from olefin containing feedstock and lower alkyl alcohols by etherification and olefin conversion and interconversion reactions.
It is another object of the present invention to provide a process for the production of liquid fuels of enhanced octane value containing MTBE and TAME.
A further object of the instant invention is an integrated liquid fuels process wherein the etherification reaction is conducted with excess alcohol but which eliminates the need to recycle excess alcohol in the etherification reactor.
Yet another object of the instant invention is to provide an integrated liquid fuels production process wherein excess alcohol and olefins are converted to an isoalkene rich etherification recycle stream.
A further object of the present invention is to provide a liquid hydrocarbon integrated process incorporating etherification with oxygenates and olefin conversion wherein valuable isoalkenes in hydrocarbon feedstream are etherified prior to the conversion reaction.